This invention relates to apparatus which aids in evaluating and aligning the RF (radio frequency) and IF (intermediate frequency) circuits of television receivers.
The performance of modern television receivers, particularly color receivers, is greatly influenced by the overall amplitude-frequency response of the system. The color television broadcast system depends upon having a selected set of signal levels for the luminance, chrominance and sound signals at the video detector in the television receiver. Any alteration in the desired transmission characteristics of the system which results in upsetting the ratios of these signals can cause serious distortion or degradation in the quality of the color picture.
Ordinarily, the transmission system can transmit the luminance, chrominance and sound signals with relatively little alteration of their original ratios from the broadcaster's transmitter antenna, all the way to the mixer of the television receiver where they are translated into frequencies within the pass band of the IF amplifier. It is primarily the frequency response characteristic of the IF amplifier that determines these relative signal levels as they are detected for further processing. Consequently, the IF amplifier response characteristic is carefully specified and stringently controlled in television receivers designed for high quality color picture reproduction.
Severe demands on the IF amplifier response characteristic are imposed by the requirement that the IF pass band provide the principle selectivity of the receiver and therefore be capable of eliminating interference from the adjacent channel picture carrier and the co-channel and adjacent channel sound carriers. The high attenuation required at these frequencies is usually provided by a number of trap circuits. Television broadcast standards result in the frequency of these trap circuits being relatively close to the band edges of the desired picture channel. Thus, rather precise tuning of the trap circuits is generally necessary to achieve the required attenuation without disturbing the desired IF pass band characteristic. Furthermore, the tuning of the trap circuits may affect the delay characteristic within the IF pass band, which could also result in degradation of the color picture. Therefore, proper tuning of the trap circuits can be important to assure maximum color picture quality, even if adjacent channel interference is not anticipated in the broadcast reception area.
While the ratios of luminance, chrominance and sound signals at the picture detector are primarily determined by the IF response characteristic of the television receiver, it is possible for these ratios to be upset by other elements of the broadcast transmission system, such as multipath, antenna or cable response and RF tuner response. Therefore, it may be desirable in analyzing and diagnosing problems in a color television system to be able to quickly verify that the receiver response is proper.
Prior art methods of verifying and aligning television receiver frequency response require the need for oscilloscope as well as sweep generator and test signal generating equipment. The signal generator equipment supplies either a swept RF signal at the antenna terminals or a swept IF signal which is normally applied to the mixer input, which encompasses the frequency band of a television channel. The oscilloscope equipment is connected to the video detector output to display a curve which corresponds to the response versus frequency. In order to identify on the cathode ray tube of the oscilloscope equipment the frequencies of interest at which the output levels are to be verified or adjusted, some means of adding "markers" is required. This may be done with a separate instrument or provision for markers may be included in the sweep generator equipment. It is also necessary to disable the receiver's automatic gain control system (AGC) so that the amplifier gain will not vary during the sweeping interval and distort the response characteristic. This is usually done by "clamping" the AGC bus at some voltage corresponding to a typical received signal level, or at a level specified by the receiver manufacturer. A variable voltage source is required for this and may be provided by a separate power supply or built in as a convenience feature of the sweep generator equipment.
From the above, it can be seen that verification and alignment of a television receiver response requires the use of a multiplicity of sophisticated instruments and, in accordance with prior art techniques, considerable interconnection of the equipment involved directly to the internal circuitry of the receiver. This requires the tedious removal of the receiver chassis from its cabinet and location of connecting terminals on the chassis by reading parts of the maintenance manual for the particular receiver involved. Because of the required skill and detailed knowledge of the receiver construction, frequency response evaluation and alignment is usually undertaken, only when other remedial efforts have failed and unacceptable picture degradation is judged to have resulted from improper frequency response. Very often a poor picture will be tolerated or improvement will not be offered by the service technician because he cannot quickly verify the need for alignment or does not wish to remove the receiver from the home to a service shop where the instrumentation is available. Thus, it can be seen that a means for simply and quickly verifying and aligning the receiver response would be of great value in the maintenance and servicing of color television receivers.
Also, before a receiver is returned from the service shop to the customer, it would be useful to have a rapid means of checking the unit for proper alignment. This is particularly true of a color television chassis which is removed from the cabinet and brought to the service shop without the picture tube. The set is checked out with a "test jig" which includes a cathode ray tube which substitutes for the units own picture tube. However, because of the cables and adapters used and the resulting possible mismatching, it is not possible to make an evaluation of receiver performance by judging picture quality on a test jig.
Consequently, it is an object of this invention to provide a method and a means for verifying and evaluating the frequency response of a television receiver without necessity of making any internal connections to the receiver.
It is also an object of this invention to provide a method and means for checking the alignment of a television receiver without making any connections to the internal circuits of the receiver, with no more knowledge of the receiver construction than the location of the antenna terminals, and to make it possible for alignment to be carried out with no more knowledge of the receiver construction than the location of the various adjustable tuning elements of the amplifier system.
It is a further object of this invention to make possible the display, evaluation and alignment of a television receiver frequency response under dynamic signal conditions with the receivers AGC circuits fully operative.